1. Field of the Invention
The present invention relates to novel synthetic precipitated silicas and to a process for producing a synthetic precipitated silicon dioxide having a new and improved combination of properties and characteristics. More particularly, the invention relates to the production of precipitated high structure silicas produced by reacting aqueous alkali metal silicate solutions with an acidification agent. The novel products are high structure finely divided silicas having certain unique properties particularly with respect to structure index, oil absorption, void volume, surface activity, friability, wetting characteristics, and further properties. The novel products are especially suitable for use as reinforcing agents in rubber, in paints, paper, detergents, molecular sieves, and in polymeric compositions.
2. Description of the Prior Art
As known in the art, finely divided silica or silicon dioxide particulates can be prepared by the acidulation of an aqueous silicate solution with an acid, such as sulfuric acid. Such products are commercially available and are characterized by, and have, the following properties: high structure, high wet cake moisture content, high oil absorption, low valley abrasion, high surface area, and low pack density. Because of properties such as high oil absorption, the pigments have been successfully used as reinforcing pigments in rubber. However, the high wet cake moisture content is disadvantageous in that the drying and filtration rates are decreased. Further, the aforementioned properties of said known and commercially available silicas render them unsuitable for many uses.
In this regard and generally speaking for the moment, known processes including those described in the literature as well as those techniques employed in the industry produce products suitable for use as reinforcing fillers. Thus, in U.S. Pat. No. 2,940,830 which issued June 14, 1960 to F. S. Thornhill, there is described a process for preparing finely divided silicas which are suitable as reinforcing agents in rubber compositions. Thornhill more specifically describes a process of preparing a silica material which is characterized by having an average ultimate particle size of 0.015 to 0.04 micron and a surface area of 25 to 200 square meters per gram by the controlled rate of addition of acid to an alkali metal silicate wherein the resultant slurry is constantly maintained at a pH above 7 in order to achieve the aforementioned end product characteristics. The Thornhill patent is specifically directed to the production of a product suitable as a reinforcing agent in rubber compositions.
In U.S. Pat. No. 3,235,331, which issued Feb. 15, 1966 to Nauroth et al., there is described a process for producing a precipitated silica which is also stated to be useful as a reinforcing agent for rubber. More specifically, this patent discloses a process wherein an aqueous alkali metal silicate solution and acid are simultaneously added to a reaction vessel. In the Nauroth et al. patent, it is pointed out that this simultaneous addition is continued until the viscosity of the pool rises through a maximum and then falls to a substantially lower value. The amount of the acidification agent and the alkali metal silicate are proportioned so as to maintain the pH of the resulting slurry substantially constant throughout the major portion of the reaction and in the range of about 10 to 12. The process is generally conducted at a temperature of 80.degree. to 90.degree. C. and the end product, after drying, results in a silica which may have a surface area of 260 square meters per gram. The patentees point out that the product is satisfactory as a reinforcing agent for rubber.
In U.S. Pat. No. 3,445,189 issued May 20, 1969 to Maat et al., there is described a process for producing finely divided silicic acid by simultaneously adding solutions of an alkali silicate and a strong mineral acid to water at a temperature between 70.degree. C. and 90.degree. C. while maintaining the reaction pH between 7 and 9. The patentees point out that the product obtained by the aforementioned process is a finely divided nongelatinous silicic acid which is useful as a filler for natural and synthetic rubber and other elastomers. It is also disclosed in this patent that for a silica to be useful as a filler for natural and synthetic rubber and other elastomers, its surface area and oil absorption are of vital importance. This patent further discloses that extensive investigations have further indicated that if a finely divided silicic acid is to have good reinforcing properties for rubber, it must have a surface area of 100 to 250 m.sup.2 /g and an oil absorption of more than 2 cc/g or 200 cc/100 g. See column 2, lines 18 through 22.
In U.S. Pat. No. 3,730,749, which issued May 1, 1973 to James E. Morgan, there is disclosed a process for preparing silica for use in reinforcing compositions. It is pointed out in Morgan that the viscosity increase which occurs during the acidification or neutralization of aqueous alkali metal silicate is substantially minimized by adding a controlled amount of an alkali metal silicate. In Examples I, II, and III of this patent, it is also noted that the silica filter cakes had solid contents of 18.5; 24.9; and 25.1 percent, respectively. This means that the percent wet cake moisture of the silicas disclosed in Examples I, II, and III is one hundred minus the percent solid content in the filter cake. In other words, the percent wet cake moisture (% WCM) of silicas mentioned in Examples I, II, and III is 81.5; 75.1; and 74.9, respectively. The surface area, the average ultimate particle sizes, and rubber data of silicas produced by the teachings of Examples II and III are listed in Table 3 which also sets forth that rubber compositions incorporating the silicas of Examples II and III have desirable rubber properties. It is further substantiated by this patent that rubber properties of silicas are related to the high wet cake moisture of the silica pigment. Thus, it is taught that a silica of high percent wet cake moisture and suitable particle size and surface area has better rubber properties than the corresponding material of lower wet cake moisture. Thus, the silicas disclosed in Morgan have a higher structure index, and therefore the silicas are useful rubber reinforcing fillers.
From the above it will be seen that the structure index of a silica is related to the rubber properties -- a silica of higher structure index will have better rubber properties than a silica of lower structure index. At this point and before turning to the remarkable concept of the present invention, the various types of synthetic silicas, as well as "structure" and "structure index" should therefore be discussed.
In this regard, and as known and accepted in the art, commercially available synthetic silicas are derived either by a liquid phase or a vapor process. Silicas obtained by the vapor process are called fumed or pyrogenic silicas. Products obtained by the liquid process are categorized as silica gels and precipitated silicas. Thus, there are three distinct types of synthetic silicas on the market:
1. Pyrogenic Silicas
Pyrogenic or fumed silicas are prepared by reacting silicon tetrachloride vapor with oxygen and hydrogen gas at high temperatures. These products have high external surface areas and differ from other silicas (e.g., gels, precipitated silicas) prepared from the liquid phase process. Cabot and DeGussa are two suppliers of pyrogenic silicas.
2. Silica Gels
Silica gels are of two types -- hydrogels and aerogels. Hydrogels are prepared by reacting a soluble silicate such as sodium silicate with strong sulfuric acid. The gel is washed salt-free, dried, steam micronized, and then classified. Aerogels are prepared from crude hydrogels by displacing its water content with an alcohol. The alcohol is then recovered by heating the gel in an autoclave.
Aerogels are lighter and fluffier than hydrogels because the shrinkage of the gel structure is avoided during the drying process. Gels have very large surface areas, generally in the range of 300-1,000 m.sup.2 /g and high porosities. Silica gels are offered, e.g., by W. R. Grace and Company under the trademark "Syloid;" by Monsanto under the trademark "Santocel;" and Glidden under the trademark "Silcron."
3. Precipitated Silicas
Precipitated silicas are produced by the de-stabilization and precipitation of silica from soluble silicate by the addition of a mineral acid and/or acidic gases. The reactants thus include an alkali metal silicate and a mineral acid, such as sulfuric acid or an acidulating agent such as CO.sub.2.
When the acidification agent is added to the alkali metal silicate, at a certain point during the process, the silica starts precipitating. The addition of the acidification agent is continued until the M.sub.2 O of the alkali metal silicate (M being the alkali metal) of the ultimate silica is less than about 1% by weight. Thus, as a general rule, the acidification agent is added to the alkali metal silicate to neutralize the alkali portion bound to the silicate anion. The reaction slurry is filtered and washed free of reaction by-product, which is the alkali metal salt of the acidification agent. The filter cake is dried and milled to obtain a silica of desired degree of fineness.
Prior to the drying step the silica filter cake generally results in a filter cake which contains a surprisingly high amount of water. For example, a silica which is useful as a filler for reinforcement of rubber and elastomers generally contains 80% to 85% water in its cake. For example, see Example No. I, U.S. Pat. No. 3,730,749 where the % wet cake moisture is 81.5. The percent water present in the filter cake is known as percent wet cake moisture or generally abbreviated at "% WCM." One hundred minus the % WCM gives the solid content of the filter cake, i.e., the amount of silica which can be recovered in the solid form upon drying the filter cake. The percent solid content of the filter cake is termed percent filter cake solids and generally abbreviated as "% FCS." Thus, % WCM and % FCS are related by the equation:
% WCM = 100 -- % FCS or PA1 % FCS = 100 -- % WCM PA1 100 pound filter cake = 85 pounds water + 15 pounds dry silica = 85% WCM + 15% FCS
If we know the value of % WCM, we can calculate % FCS or vice versa.
Thus, a silica filter cake having 85% WCM will have 100 minus 85 or 15% FCS. This means that 15 pounds of silica can be recovered from such a filter cake by evaporating or drying 85 pounds of water from one hundred pounds of filter cake. The total weight of filter cake consists of water and solid silica. In the example where the % WCM is 85, one can recover only 15 pounds of solid silica as can be seen below:
Thus, there are 85 pounds of water associated with 15 pounds of solid silica content or 85/15 .times. 100 = 567 of water per 100 pounds of solid silica.
The water associated with the silica content of filter cake is structural water. This water is present whereby it occupies the available space between the silica aggregates and also the space inside the silica aggregates. As used herein, the term "structure" is defined as the ability of a silica to hold water in its wet cake. When silicas, such as the aforementioned known prior art products, hold a high percentage of water, i.e., from about 70 to 85%, they are known as high structure silicas. Materials holding less than 70% or from about 50 to 70% are referred to as low structure silicas. This total structural water content is a very important property of silica and is directly related to the functional and end use properties of silica. The amount of total structural water associated with 100 pounds of solid silica content of the filter cake is defined as "structure index" and abbreviated as S. I.
Mathematically, structural index (S. I.) of silica can be calculated if either the % wet cake moisture (WCM) or the % filter cake solid (FCS) values of said silica are known: ##EQU1## Structure index of silicas in wet cake moisture range of 80-85% is listed in Table I.
Table I ______________________________________ Structure Index of Silicas With % WCM of 85 - 80 % WCM 100 - % WCM S. I. ______________________________________ 85 15 567 84 16 525 83 17 488 82 18 455 81 19 426 80 20 400 ______________________________________
Prior art precipitated silicas such as disclosed in the aforementioned patents (see U.S. Pat. No. 2,940,830; 3,235,331; 3,445,189; 3,730,749) are high structure silicas having high S. I. values. As stated, these silicas are useful as reinforcing fillers in elastomers and rubber. As a final point in this discussion of the prior art, it may be noted that in addition to the aforementioned high structure reinforcing silicas, there has recently been developed a new class of low structure silica compounds which though unsuitable as reinforcing fillers, have utilities for use in specific areas such as polishing and cleaning agents in dentifrices. Specific examples of such new products and methods for their production are disclosed in U.S. patent applications Ser. Nos. 286,655, filed Aug. 6, 1972 (now allowed) and 472,580, filed May 22, 1974.